Take the content of a list and append it to another list

extend() vs append(): What's the difference?

Here's the key difference between extend() and append(): The former adds each item of the input iterable to the list, while the latter adds the entire input as a single item.

Think of it as inviting a group of friends to the party (extend) vs inviting a single friend (append). Everybody loves a party!

Memory, Lists, itertools.chain, oh my!

For large data or merging multiple lists, itertools.chain() proves itself invaluable, like a Swiss Army knife of the Python world.

import itertools

list1 = [1, 2, 3]
list2 = [4, 5, 6]
combined = itertools.chain(list1, list2)


combined is now a memory-friendly iterator over list1 and list2. Yes, the party just got bigger and nobody tripped the circuit breaker!

Conditional concatenation sans loops

What if you only want to invite some friends from list2 to the party? The list.extend() method combined with a generator expression will save the day!

list1 = [1, 2, 3, 4, 5]
list2 = [10, 20, 30, 40, 50]
list1.extend(x for x in list2 if x > 25)
print(list1)  # Outputs: [1, 2, 3, 4, 5, 30, 40, 50]


Mind the performance!

When dealing with large datasets, remember to regularly check performance like a doctor checks a heartbeat. The list1 += list2 syntax is neat but may not outperform list1.extend(list2) when the guest list (elements) is in the thousands.

Prioritize readability and maintainability

A complex solution may seem cool but give priority to readability and maintainability. A simpler for-loop could be better than a perplexing one-liner. Remember, coding is communication first with other humans, then with the machine!

Memory use and large-level operations

Using generators or iterators when merging lists can save a lot of memory—imagine it like fitting more guests in your party without renting a bigger hall!

Conditional content appending: Let's be picky

Sometimes we need to append content based on various conditions. Cue list comprehensions:

list1 = [1, 2, 3, 4]
list2 = [5, 6, 7]

# Append only even numbers from list1 to list2
list2 += [x for x in list1 if x % 2 == 0]


This gets your peer-reviewed, beautifully optimized solution, all while keeping the readability intact. And remember, hit the official documentation when in doubt!